If actions were seperate xml files rather than listed in a single /etc/xdg/Thunar/uca.xml or ~/.config/Thunar/uca.xml file, then other applications could install actions much the same way installing gimp creates /usr/share/applications/gimp.desktop which xfce reads and creates a menu item for.

Some examples of uses.
If your audio player could install actions for "play now" or "enqueue" as opposed to being limited to the current "open with".  Or Inkscape could install an action that launches a dialog to convert svgs to other rasterized formats.